The present invention relates to a pre-loaded piezoelectric actuators using a piezoelectric element. Preloading, by use of a coil spring for example, applies compressive force to the piezoelectric element to enhance the length of drive stroke of a movable work piece, positioning accuracy piece and the operating frequency range.
In applications of piezoelectric actuators in piezo-press systems in which a piezoelectric element is used to drive a work delivering drive stroke displacement for punching or forming metal, for example, several actuator characteristics are important. In high-speed press applications, for example, a piezoelectric actuator may be used to provide output forces as large as several hundred kilograms to several tons and in processing a sheet of metallic foil of a thickness such as 10 .mu.m, work piece positioning accuracy of 1 .mu.m may be required. In such applications, important actuator characteristics include:
effective high-force work piece drive; PA1 accuracy and repeatability of work piece positioning; PA1 length of work piece drive stroke; PA1 work piece drive repetition frequency; PA1 adjustability of work piece positioning; PA1 ease of actuator installation, etc. PA1 (a) providing a form of piezoelectric actuator unit including a form of laminated piezoelectric element whose axial length may be shortened by application of compressive force, a work piece, a support device, and spring means for resiliently holding the work piece and piezoelectric element against an end member of the support device; PA1 (b) determining the amount of compressive force necessary to produce substantially the maximum shortening of the axial length of the piezoelectric element when subjected to non-destructive compressive force; PA1 (c) providing spring means able to exert a pre-loading force of substantially the amount of compressive force as determined in step (b); and PA1 (d) assembling the form of piezoelectric actuator unit as described in step (a) incorporating the spring means as provided in step (c) to provide a pre-loaded piezoelectric actuator unit which is substantially free of work piece displacement loss due to piezoelectric element compression under load.
This application, which is a continuation-in-part of application 07/522,183, filed May 11, 1990, now U.S. Pat. No. 5,095,725 is particularly directed to piezoelectric actuators as generally illustrated in FIGS. 1, 2 and 3, which are pre-loaded so as to provide improved performance in areas such as the characteristics listed above. Pre-loading in accordance with the invention is more particularly described below under the heading "High Performance Piezoelectric Actuators."
Conventional press machines or systems generally use a die closing, wedge-type or toggle-type mechanism for die opening and closing, with the die-closing mechanism being driven and operated by a hydraulic cylinder or an air cylinder.
Further, specialty-parts assembling machines and assembling jigs also generally use a hydraulic cylinder or an air cylinder as the actuator to attach parts brought into the jig, as in the above case.
The above-mentioned conventional presses that are generally used have, in addition to the complexity of their die-closing mechanism, the problem that when using electric signals to control the presses, it is difficult to synchronize the operation of the mechanism of the main body of the press to the die-closing mechanism because the die-closing mechanism is operated by a hydraulic cylinder or an air cylinder.
Generally, press systems or mechanisms using piezoelectric elements had the following problems:
1. Because the die was a fixed-stripper type, warping occurred while shearing the material to be processed. Further, an actuator using a piezoelectric element only operates compressively and breaks down in tension making it impossible to drive the die set.
2. Since the structure is such that an actuator using a layer-type piezoelectric element directly drives a punch, its dismantling and replacement when damaged or degraded consumed a great deal of time.
3. Since the actuator using a layer-type piezoelectric element was mounted directly on a die, the actuator had to be installed on another die to process parts of a different shape.
4. The actuators or dies using a piezoelectric element required a special device to mount and hold them.
5. Because the conventional actuator using a piezoelectric element was not installed as an independent device, a replacement of the actuator and the movable piece required a great amount of time.
6. Because the insulation between the case and the actuator used an air space, a hazardous electrical shock could have been caused by a short circuit to the case during operation.
7. It was impossible to measure length of stroke of the movable piece.
8. In assembling the layer-type piezoelectric element and the movable piece into the drive mechanism, a space was created between the movable piece and the movement-receiving end of the drive mechanism because of insufficient processing precision. As a result, the drive mechanism became incapable of performing the desired operation.
9. Because the case for the piezoelectric element was made of a material having a larger thermal expansion coefficient than that of the piezoelectric element, the case expanded slightly in size during operation and caused malfunctions.
10. The case for the piezoelectric element was deformed by a high-load output and became incapable of reproducing the stroke length setting at no load.
11. The movable piece in the actuator, which used ferrous material, was compressed by the high loading and resulted in a reduction of the length of the stroke.
12. Because some conventional configurations had the layer-type piezoelectric element arranged in series with the resilient element that applies a compressive force constantly to the piezoelectric element, the length of the piezoelectric actuator needed to be longer than that of the layer-type piezoelectric element, thus limiting the compactness of the piezoelectric actuator.
The present invention was made in light of the above problems Its first purpose is to provide an actuator using materials and parts that will allow simplified construction and that will make the actuator able to respond speedily to the operation and able to synchronize its motion directly from electric signals.
The second purpose of the present invention is to enhance processing precision and durability of a relatively low-cost press with replaceable parts that can be easily dismantled.
The third purpose of the present invention is to provide an actuator using a piezoelectric element which is easily handled and of high precision.
The fourth purpose of the present invention is to provide an actuator using a piezoelectric element which is compact, of high precision, and reliable.